There is an urgent need to develop new means for potentiating protective immune responses against pathogens that infect the oral, gastric and urogenital mucosae. The objective of this proposal is to evaluate the mucosal adjuvant activities of LT-IIa and LT-IIb, two Escherichia coli Type II enterotoxins. LT-IIa and LT-IIb induce distinctive patterns of enhanced immune responses which are profoundly different from those induced by cholera toxin (CT). Whereas CT commonly induces a predominant Th2-type response based on antibody isotype and cytokine patterns, mice administered with Type II enterotoxins as adjuvants exhibit a more balanced Th1/Th2 response. We have also demonstrated that LT-IIa, LT-IIb, and CT interact with different populations of lymphocytes and induce in those populations distinctive cellular responses (apoptosis, cytokine production, proliferation, etc.). Collectively, these data provide strong evidence that LT-IIa and LT-IIb (and CT) utilize different cellular and molecular mechanisms for immunomodulation. Our hypothesis is that the distinctive adjuvant activities of LT-IIa and LT-IIb (and CT) are elicited by their binding affinity for different receptors on immunocompetent cells which are required to trigger specific signal transduction events. To test this hypothesis, the adjuvant activities of the LT-IIa and LT-IIb, and a collection of mutant enterotoxins with altered receptor-binding activities, will be analyzed in a mucosal mouse model using Agl/II of the oral pathogen Streptococcus mutans as a model antigen. Other LT-IIa and LT-IIb mutants will be engineered to establish the importance of ADP- ribosylation and cellular trafficking in immunomodulation. Prior investigations have revealed a receptor on lymphocytes which is likely the trigger for the adjuvant activities of LT-IIb. Ablation and blocking experiments using relevant lymphocytes will be used to characterize the receptor. The affect of the enterotoxins on the cellular and molecular responses of dendritic cells, the major sentinel antigen- presenting cells of the mucosa, will be investigated as a further means to correlate the adjuvant activities of LT-IIa and LT-IIb with particular lymphocytes. Efficacy of the mutant enterotoxins as protective mucosal adjuvants will also be determined using an established S. mutans murine colonization model. The fundamental information obtained herein will be essential for establishing the potential of Type II enterotoxins, or their non-toxic mutants, as mucosal adjuvants for subsequent vaccine use.